PHYSICAL FACTORS IN PLANT DISTRIBUTION 55 



The evolutionary factor in distribution presents its own prob- 

 lems, but they are so difficult and at present so far from any 

 hopeful prospect of solution that we must be content to let them 

 alone. Who is there among us who is willing to state why the 

 C'actaceae originated in America and not in Africa, when we 

 are not even certain as to the nearest living relatives of this 

 family? Who will state why the graminean stock has become 

 so widel}" disseminated and has colonised such dissimilar habi- 

 tats and climates without exhibiting a greater morphological 

 differentiation? 



The role of the evolutionary factor can not be escaped in any 

 consideration of distribution. We owe it to the historj^ of the 

 great plant stocks that Yucca arhorescens occurs in the Mojave 

 Desert and that Aloe dichotoma, somewhat similar to it in form, 

 grows in the deserts of South-west Africa. It is likewise a part 

 of phylogenetic history that very many plants were formerly 

 confined to particular regions whereas they have now been intro- 

 duced over seas into climates which prove to be wholly con- 

 genial to them. However, the ranges of Yucca and Aloe in their 

 respective continents are to be sought in the operation of the 

 environment, and the spread of an introduced plant in a new 

 continent is controlled by physical factors just as it was in its 

 native continent. 



Evolutionary activities' and phylogenetic history are to be 

 thought of as supplying the raw materials out of which the physi- 

 cal environment has made the present distributional complex 

 of the earth's surface. We can not hope at present to under- 

 stand why we have a strong development of the genus Yucca 

 in the southwestern United States and a rich development of 

 Aloe in South Africa, and the answer to such a question, if forth- 

 coming, would have only a remote relation to the study of the 

 present influences which are limiting the distribution of the 

 members of these two stocks in their respective contients. 



The role of the paleoclimatic factors is also an immanent one 

 in the determination of plant distribution. Many species have 

 a known ancestry and a known ancestral range, at the same 

 time that they now inhabit areas of such small size that it is 



